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Properties of pure substances - Basics By Udit Gupta Mechanical Engineering, MNNIT Allahabad Ex RIL Manager Jamnagar, SEZ refinery GATE qualified 2013,2015
Need to study Properties and the behaviour of substances are very important for our studies of device and thermodynamic systems. For example in a steam power plant, we must know the properties of water to properly size equipment such as the burners or heat exchangers turbine, and pump for the desired transfer of energy and the flow of water. As the water is transformed from liquid to vapour, we need to know the temperature for the given pressure, and we must know the density or specific volume so that the piping can be properly dimensioned for the flow If the pipes are too small, the expansion creates excessive velociti leading to pressure losses and increased friction, and thus demanding a larger pump and reducing the turbine's work output.
Pure Substance A pure substance is one that has a homogeneous and invariable chemical composition It may exist in more than one phase, but the chemical composition is the same in all phases. Thus, liquid-water, a mixture of liquid water and water vapour (steam), and a mixture of ice and liquid water are all pure substances; every phase has the same chemical composition. In contrast, a mixture of liquid air and gaseous air is not a pure substance because the composition of the liquid phase is different fro that of the vapour phase. Exception!! Air is treated as a pure substance though it is a mixture of gases
Vapour-liquid-solid phase equilibriunm Consider as a system 1 kg of water contained in the piston/cylinder arrangement shown in Fig. (a). iquid water Suppose that the piston and weight maintain a pressure of 0.1 MPa in the cylinder and that the initial temperature is 20 C. As heat is transferred to the water, the temperature increases appreciably, the specific volume increases slightly, and the pressure remains constant. Water vapor When the temperature reaches 99.6 C, additional heat transfer results in a change of phase, as indicated in Fig. (b). That is, some of the liquid becomes vapour, and during this process both the temperature and pressure remain constant, but the specific volume increases considerabl When the last drop of liquid has vaporized, further transfer of heat results in an increase in both the temperature and specific volume of the vapour, as shown in Fig. (c). Water vapor
Vapour-pressure curve The term saturation temperature designates the temperature at which vaporization takes place at a given pressure. This pressure is called the saturation pressure for the given temperature. Thus, for water at 99.6 C the saturation pressure is 0.1 MPa, and for water at 0.1 MPa the saturation temperature is 99.6 C. For a pure substance there is a definite relation between saturation pressure and saturation temperature. A typical curve, called the vapor-pressure curve, is shown. va Temperature
Saturated and subcooled liquid If a substance exists as liquid at the saturation temperature and pressure, it is called a saturated liquid If the temperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid (implying that the temperature is lower than the saturation temperature for the given pressure) or a compressed liquid (implying that the pressure is greater than the saturation pressure for the given temperature).
Quality When a substance exists as part liquid and part vapour at the saturation temperature, its quality is defined as the ratio of the mass of vapour to total mass. Thus, in Fig. (b), if the mass of the vapor is 0.2 kg and the mass of the liquid is 0.8 kg, the quality is 0.2 or 20%. The quality may be considered an intensive property and has the symbol x. Note: Quality has meaning only when the substance is in a saturated state, that is, at saturation pressure and temperature.